In automatic transmissions for vehicles, it is common for fluid flow to be restricted by an orifice. Orifice plates and orifice plugs are two common methods of doing this when it is not possible to provide the orifice by drilling or reaming the desired hole diameter into the transmission case. Page 325-4L-17 of the 1982 Buick Regal--Le Sabre--Electra--Riviera Chassis Service Manual shows the procedure for removal of a cup plug from a transmission case where it is part of a lo-reverse overrun clutch circuit.
In this specific application, an orifice plate and a seal have been used between the transmission case and a transmission case extension to restrict the flow of fluid to a yoke bushing. The restriction is necessary to prevent transmission fluid from escaping past a yoke seal to the outside of the transmission. A continued loss of fluid could lead to the eventual loss of transmission line pressure. The restriction is also necessary to maintain the desired rate of transmission fluid flow to other transmission component interfaces. Excess flow of transmission fluid to the yoke bushing would reduce the rate of transmission fluid flow to the other interfaces.
The particular disadvantage of the orifice and seal combination as employed in this system is in the difficulty of assembly. The plate and seal must be dropped into a bore in the transmission case shortly before assembling the transmission case to the transmission case extension. Grease has been used to hold the plate and seal in position during assembly. Even with the use of grease, the parts can fall out of place during assembly. A cup plug with an orifice could be used instead of the plate to overcome the potential mispositioning of the plate, but the seal could still be mispositioned. The seal would be ineffective in preventing fluid leakage between the transmission case and the transmission case extension if mispositioned. Leakage could potentially reduce the flow of oil to the yoke bushing to the point where it may be insufficient to prevent a seizure of the yoke to the yoke bushing.